Optical fibers are well known in the art and useful for many applications in modern communications systems. A typical fiber optic communications system, for example, is shown schematically in FIG. 1A, comprising a source of optical signals 10, a length of optical fiber 12 coupled to the source, and a receiver 14 coupled to the fiber for receiving the signals. One or more amplifying systems 16a, 16b, may be disposed along the fiber for amplifying the transmitted signal. Filters are useful in these systems to change the power levels of various signals, especially in wavelength division multiplexed systems, along with signal modulation and wavelength routing.
Basically, the optical fiber 12 shown in FIG. 1A comprises an inner core fabricated from a dielectric material having a certain index of refraction, and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. Since in the conventional optical fiber, light is confined mostly in the core region, the ability to externally effect propagation behavior of the light in the fiber is significantly limited. With conventional fibers, to change the propagation behavior of light in the core, one is essentially limited to the application of strain and/or temperature changes to the fiber.
Optical fiber gratings including Bragg and long-period gratings are important elements for selectively controlling specific wavelengths of light within an optical fiber. A typical Bragg grating comprises a length of optical fiber including a plurality of perturbations in the index of refraction substantially equally spaced along the fiber length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. Light of the selected wavelength .lambda. is reflected back to point of origin, and the remaining wavelengths pass essentially unimpeded. Such Bragg gratings are useful in a variety of applications including filtering, stabilizing semiconductor lasers, reflecting fiber amplifier pump energy, and compensating for fiber dispersion.
Bragg gratings in optical fibers are conveniently fabricated by providing a fiber having a core doped with one or more materials sensitive to ultraviolet light, such as a fiber having a core doped with germanium oxide, and then exposing the fiber at periodic intervals to high intensity ultraviolet light from an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce perturbations in the index of refraction. The appropriate periodic spacing of the perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A difficulty with conventional fiber Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects light in only a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However, in many applications, such as multiplexing, it is desirable to have a grating whose wavelength response can be tuned, that is, controllably altered. A tunable fiber grating has been attempted with use of a piezoelectric element to strain the grating. See Quetel et al., 1996 Technical Digest Series, Conf. on Optical Fiber Communication, San Jose, Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120, paper No. WF6. However, the strain produced by piezoelectric actuation is relatively small, limiting the tuning range of the device. Moreover, piezoelectric activation requires a continuous application of relatively high voltage, e.g., approximately 100 volts for 1 nm strain. Another approach for providing a tunable Bragg grating involves use of thermally-induced strain on the fiber, as described in U.S. application Ser. No. 08/957,953, "Device for Tuning Wavelength Response of an Optical Fiber Grating," filed Oct. 27, 1997 by Fleming et al. (the '953 application), and assigned to the present assignee, which is incorporated herein by reference. The '953 application involves use of a temperature-sensitive body attached to the exterior of the optical fiber adjacent the Bragg grating region.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance .LAMBDA. which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA. which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA..gtoreq.10.lambda.. Typically .LAMBDA. is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5.LAMBDA. to 4/5.LAMBDA.. In some applications, such as chirped gratings, the spacing .LAMBDA. can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the fiber core, long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, it is a cladding mode. The spacing .LAMBDA. of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA. can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped to provide a wavelength dependent loss.
Long-period grating devices are thus useful as filtering and spectral shaping devices in a variety of optical communications applications. Each long-period grating with a given periodicity (.LAMBDA.) selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. This wavelength is determined by .lambda..sub.p (n.sub.c -n.sub.cl).multidot..LAMBDA., where n.sub.g and n.sub.ng denote the effective index of the core mode and the cladding mode, respectively. The values of n.sub.c and n.sub.cl are dependent on the relative values of the refractive indices of the core, cladding, and air. A difficulty with conventional long-period gratings, however, is that their capability to equalize amplifier gain is limited, because they filter only a fixed wavelength acting as wavelength-dependent loss elements. Thus, there is a need for a long-period grating whose transmission spectrum can be controlled. It is desirable to have a tunable (or reconfigurable) long-period grating which, upon activation, can be made to dynamically filter other wavelengths (i.e., besides .lambda..sub.p) Further, it is desirable to be able to selectively filter a broad range of wavelengths, e.g., for efficient operation of multiple-channel WDM in telecommunication systems. A recent device for providing a tunable long-period grating is described in U.S. application Ser. No. 08/957,956, "Tunable Long-Period Grating Device and Optical Systems Employing Same," filed Oct. 27, 1997 by Jin et al. (the '956 application), and assigned to the present assignee, which is incorporated herein by reference. The '953 application involves use of a strain-inducing body secured to the fiber adjacent the grating region for changing the spacing between the perturbations of the grating.
As may be appreciated, those concerned with the development of optical communications systems and, more particularly fiber devices, continually search for new components and fiber designs. As optical communications systems become more advanced, there is growing interest in increasing the number of wavelengths that may be transmitted by the systems and therefore in new methods and devices for modulating, filtering, and switching wavelength channels. The instant invention provides a new structure for a tunable optical fiber device, including a tunable Bragg or long-period grating device, that does not involve complicated structures or application of strain to the fiber.